The present invention relates to an electrophotographic photoreceptor for use in electrophotography.
Amorphous silicon containing hydrogen H (to hereinafter be referred to as a-Si:H) has received a great deal of attention as a photoconductive material, and has been used in a variety of applications, such as solar cells, thin film transistors, image sensors, and electrophotographic photoreceptors.
Materials used as photoconductive layers in conventional electrophotographic photoreceptors can be categorized as either inorganic (e.g., CdS, ZnO, Se, or Se-Te) or organic (poly-N-vinylcarbazole (PVCZ) or trinitrofluorene). The a-Si:H photoconductive material has many advantages over the above-mentioned conventional organic and inorganic materials, such as that it is non-toxic and does not require recovery, high spectral sensitivity in the range of visible light is effected, and its high surface hardness ensures high resistance to wear and good anti-impact properties. It is for these reasons that a-Si:H is receiving a great deal of attention as a promising electrophotographic photoreceptor.
The a-Si:H material has been developed as an electrophotographic photoreceptor on the basis of the Carlson system. In this case, good photoreceptor properties mean high dark resistance and high sensitivity to light. However, since it is difficult to incorporate these two properties in a signal layer photoreceptor, a barrier layer is therefore arranged between the photoconductive layer and a conductive support, with a surface charge-retaining layer being formed on the photoconductive layer, to constitute a multilayer structure, and thereby satisfy the two requirements described above.
As a conventional barrier layer, an insulating single layer having a high resistance is used. However, if such an insulating layer is quite thick, carriers flowing from the photoconductive layer to the conductive substrate cannot pass through the barrier layer. As a result, the residual potential is increased. If, on the other hand, the barrier layer is not so thick, the layer then causes insulating breakdown to occur, due to a developing bias applied to the photoreceptor. When the barrier layer employs a p- or n-type semiconductor, if the layer has a large thickness, the carriers are trapped in structural defects such as dangling bonds and a residual potential is increased. Also, if the layer is not so thick, the carriers from the conductive substrate cannot be blocked, with the result that the charging capacity is decreased.
For use in a two-color copying machine or in a machine used as both a printer and a copying machine, a photoreceptor is required which can be charged in both positive and negative polarities. When such a photoreceptor is formed using a-Si, oxygen may be added thereto or an insulating layer may be formed between a photoconductive layer and a conductive substrate. However, in the former case, the number of defects in the film will be increased due to addition of oxygen, thus degrading the sensitivity and residual potential. In the latter case, if the insulating layer is quite thick, carriers become trapped, and the residual potential is undesirably increased. If, on the other hand, the insulating layer is not so thick, the barrier layer causes insulating breakdown to occur, due to a developing bias applied to the photoreceptor.